As data throughput requirements increase, and the amount of available spectrum decreases, carrier aggregation has emerged as an attractive option for wireless service providers. With carrier aggregation, a wider bandwidth may be achieved by utilizing multiple (typically two) component carriers, where the frequency bands centered at the component carriers need not be contiguous. For example, carrier aggregation may be employed in order to utilize both an 800 MHz carrier and an 1800 MHz carrier for transmitting data streams to a single user terminal. Because each component carrier corresponds to a different cell, and the various cells may have different coverage areas, use of carrier aggregation may increase overall coverage area, in addition to increasing throughput.
The 3rd Generation Partnership Project (3GPP) Long Term Evolution Advanced (LTE-Advanced) standard is one example of a standard that supports carrier aggregation. The LTE-Advanced standard (also known as “Release 10”) specifies a primary cell, or “Pcell,” associated with a primary component carrier (PCC), and a secondary cell, or “Scell,” associated with a different, secondary component carrier (SCC). In LTE-Advanced, system information for the Pcell is transmitted and decoded in the same way as defined under the earlier Releases 8 and 9, i.e., via a master information block (MIB) within the physical broadcast channel (PBCH), and via system information blocks (SIBs) within the physical downlink shared channel (PDSCH). Thus, the PCC is backwards-compatible with UEs configured according to Release 8 or 9. Conversely, the Scell does not transmit system information, and is not backwards-compatible with UEs configured according to Release 8 or 9. Instead, system information associated with the Scell is transmitted by the Pcell, via the PCC. Thus, in LTE-Advanced, UEs decode MIBs and SIBs transmitted on the PCC, but are not required to decode any MIB or SIB transmitted on the SCC. Similarly, paging messages are transmitted using the PCC, but not the SCC.
The base station (evolved NodeB, or eNB) of a Pcell may, in some scenarios, provide substantially worse channel conditions than the Scell, and/or the Scell may transmit signals that “shadow” the Pcell signals (a condition referred to as “power imbalance”). With respect to shadowing, for example, a UE may be much closer physically to the eNB of an Scell than to the eNB of a Pcell. As a result, in scenarios such as these, UE reception of system information and/or paging messages transmitted via the PCC may be degraded. Conventional systems tend to under-utilize the Scell by deactivating the Scell, causing the Scell to transmit power-limited signals, or causing the Scell to refrain from transmitting signals, in order to preserve Pcell signal quality. These approaches, however, tend to result in throughput loss for both the Scell eNB and the UE.